This study will elucidate the factors critical in regulating the addition of sialic acid to a mucin-type glycoprotein. Mucin-type glycoproteins are rich in mucous secretions, although the carbohydrate which is characteristic of these glycoproteins also is found in blood group substances, cell surface glycoproteins and immunoglobulins. Mucins have been shown to mask cellsurface antigens in mouse mammary tumor cells, and the metastatic ability of tumor cells has been correlated with cellular sialic acid content. The system of study, rat mammary tumor cells, contains a mucin-type glycoprotein which accounts for 80% of the protein-bound sialic acid. Two rat mammary ascites tumor cell lines, MAT-B1 and MAT-C1, were derived from the same solid tumor (13762) yet differ dramatically in the sialic acid content of the major mucin-type glycoprotein. Sialic acid metabolism in the MAT-B1 and MAT-C1 cells is being investigated through characterization of (1)\sialyltransferase, (2)\neuraminidase, (3)\CMP-sialic synthetase and CMP-sialic acid hydrolase and (4)\the types of sialic acid present in the major glycoprotein. Preliminary results on the MAT-B1 and MAT-C1 sialyltransferase indicate that: (1)\the major endogenous acceptor of cellular sialyltransferase is the major mucin-type glycoprotein; (2)\greater than 60% of the activity remains in the supernatant following centrifugation of a cell homogenate at 100,000 x g for 1 hour; and (3)\no significant differences in the MAT-B1 and MAT-C1 sialyltransferases have yet been detected. Further work will potentially uncover a regulatory mechanism which explains the difference in sialic acid content between the two cell lines. The results of this work pertain to the problem of how cells regulate the carbohydrate composition of the cell surface and of secreted glycoproteins, and may ultimately contribute to a chemical understanding of tumorigenesis and metastasis.